Method and system for adaptive control of turning operations

ABSTRACT

An adaptive control system for adaptively controlling a turning operation performed at a work piece by a turning tool adjusts a controlled input operation parameter F to maintain an output operation parameter ΔM substantially at a predetermined value ΔM o  to compensate variation of the output operation paramemeter ΔM caused by the variation of at least one operation condition B=B (t). The system comprises a sensor ( 8 ) of the output operation parameter ΔM for providing a signal U c  proportional to a current value ΔM c  and an adaptive controller ( 10 ) for determining a value F c  to which the input operation parameter F should be adjusted, as a function of kU c , where k is a signal transmission coefficient which comprises an invariant signal transmission coefficient component k o  inversely proportional to ΔM o . The adaptive controller includes an amplifier transforming the signal U c  into k o U c , and an input parameter override unit controlled to adjust the controlled operation input parameter to F c . The adaptive controller further comprises a correction processing means calculating k c U c , where k c  is a varying signal transmission coefficient component whose current values depend on the variation of the operation condition B=B (t). The adaptive controller is capable of calculating k=f (k o , k c ).

FIELD OF THE INVENTION

This invention relates to adaptive control of cutting operations onCNC-operated machine tools in which a controlled input parametercharacterizing the movement of a cutting tool relative to a workpiece,is continuously adjusted during a cutting operation in response to ameasured output operation parameter defining the productivity of theoperation. The present invention particularly concerns the adaptivecontrol of turning operations performed on lathes, where the controlledinput parameter is a feed rate of the cutting tool and the outputparameter is a cutting torque, cutting force or consumed power of thelathe's spindle drive.

BACKGROUND OF THE INVENTION

In a CNC-operated lathe, a program instructs a feeding means on a feedrate with which a tuning tool should cut a workpiece and instructs thelathe's spindle drive on a speed with which a workpiece associatedtherewith should be rotated. The feed rate and the selected speed arecontrolled input parameters that are normally fixed by the program foreach cutting operation based on pre-programmed cutting conditions suchas depth of cut, diameter of the workpiece, material of the workpiece tobe machined, type of the cutting tool, etc.

However, the efficiency of CNC programs is limited by their incapabilityto take into account unpredictable real-time changes of some of thecutting conditions, namely the changes of the depth of cut,non-uniformity of a workpiece material, tool wear, etc.

Optimization of cutting operations on CNC-operated lathes, as well as onmost other machine tools, is usually associated with the adaptivecontrol of the movement of a cutting tool relative to a workpiece and,particularly, with the adjustment of the cutting tool's feed rate as afunction of a measured cutting torque developed by the machine tool, tocompensate the change in cutting conditions.

FIG. 3 illustrates a known control system for adaptively controlling aturning operation, for use with a CNC-operated lathe having a feedingmeans and a spindle drive that are instructed by a CNC program toestablish the movement of, respectively, a cutting tool and a workpieceattached to the spindle, with pre-programmed values of respectivecontrolled input parameters F_(o) that is a basic feed of the cuttingtool and S_(o) that is a basic rotational speed of the spindle (thecutting tool and the workpiece are not shown).

As seen in FIG. 3, the control system comprises a torque sensor formeasuring a cutting torque ΔM developed by the spindle drive. Dependingon an unpredictable variation of cutting conditions B, the cuttingtorque ΔM may have different current values ΔM_(c), in accordance withwhich the torque sensor generates current signals U_(c) proportional toΔM_(c). The control system also comprises a known adaptive controllerincluding an amplifier with a signal transmission coefficient k_(o)′,transforming the signal U_(c) into k_(o)′U_(c) and subsequentlydetermining a value F_(o)/F_(o)=ƒ(k_(o)′U_(c)) to which the feed rateF_(c) should be adjusted, by a feed rate override unit, in order tocompensate the variation of the cutting conditions B and to, thereby,maintain the cutting torque ΔM_(c) as close as possible to its maximalvalue ΔM_(max), required for the maximal metal-working productivity.

The maximal value of the cutting torque ΔMmax is a predetermined cuttingtorque developed by the spindle drive during cutting with a maximaldepth of cut, and the signal transmission coefficient of the amplifieris defined as ${k_{o}^{\prime} = \frac{1}{U_{\max}}},$

where U_(max) is a signal from the torque sensor corresponding to themaximal torque ΔM_(max).

The current value F_(o)/F_(o) is defined by the adaptive controllerbased on its signal transmission coefficient k_(o)′, pre-programmedbasic feed rate F_(o) and signal U_(c), in accordance with the followingrelationship: $\begin{matrix}{{\frac{F_{c}}{F_{o}} = {A - {k_{o}^{\prime}U_{c}}}},} & (1)\end{matrix}$

where A=F_(id)/F_(o), and F_(id) is an idle feed (feed without cutting).

The coefficient A characterizes the extent to which the feed rate F_(c)may be increased relative to its pre-programmed value F_(o), and itusually does not exceed 2.

Since, as mentioned above, the signal U_(c) is proportional to thecutting torque ΔM_(c), the relationship (1) may be presented, for thepurpose of explaining the physical model of the adaptive controller, asfollows: $\begin{matrix}{{\frac{F_{c}}{F_{o}} = {{A - {K_{o}^{\prime}\Delta \quad M_{c}}} = a_{c}}},} & (2)\end{matrix}$

where K_(o)′ is a correction coefficient corresponding to the signaltransmission coefficient k_(o)′ of the adaptive controller and it isaccordingly calculated as$K_{o}^{\prime} = {\frac{1}{\Delta \quad M_{\max}}.}$

The physical model of the adaptive controller is illustrated in FIG. 4.As seen, the change of the cutting conditions B influences the currentvalue ΔM_(c) of the cutting torque which is used by the adaptivecontroller to determine the coefficient a_(c) characterizing the currentvalue F_(c) to which the feed rate should be adjusted to compensate thechanged cutting conditions B.

It is known that, in a turning operation, the cutting condition thatchanges unpredictably in time and that is mostly responsible for thevariation of the cutting torque is the depth of cut h_(c)=h_(c)(t). Whenturning a workpiece of a given diameter, the cutting torque ΔM_(c) isproportional to the depth of cut h_(c) as follows:

ΔM_(c)=cF_(c)h_(c)=cF_(o)a_(c)h_(c),   (3)

where c is a static coefficient established for turning operations anda_(c) is defined in the equation (2).

Based on the equations (3) and (2), the cutting torque ΔM_(c) may beexpressed as: $\begin{matrix}{{\Delta \quad M_{c}} = \frac{A\quad c\quad F_{o}h_{c}}{1 + {c\quad F_{o}h_{c}K_{o}^{\prime}}}} & (4)\end{matrix}$

If in the equation (4), the coefficient A=2 and h_(c)=h_(max), themaximal cutting torque ΔM_(c) may be expressed as: $\begin{matrix}{{\Delta \quad M_{\max}} = \frac{2\quad c\quad F_{o}h_{\max}}{1 + {{cF}_{o}h_{\max}K_{o}^{\prime}}}} & (5)\end{matrix}$

Similarly, when the depth of cut is of a very small value h_(min) suchthat h_(min)/h_(max)<<1, the cutting torque ΔM_(min) will also be verysmall:

ΔM_(min)≈2cF_(o)h_(min)<<ΔM_(max)   (6)

It follows from the above that, with Me adaptive controller asdescribed, there still may be a significant variation of the cuttingtorque ΔM_(c) during cutting with the depth of cut varying in a widerange, as illustrated in FIG. 5, curve I.

It is the object of the present invention to provide a new method andsystem for the adaptive control of a turning operation.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provideda method of adaptively controlling a turning operation performed on aworkpiece by a turning tool, by controlling an adjustable inputoperation parameter F of the movement of the turning tool relative tothe workpiece, to maintain an output operation parameter ΔMsubstantially at a predetermined value ΔM_(o) and thereby tosubstantially compensate the variation of said output operationparameter ΔM caused by the variation of at least one operation conditionB=B(t) varying in time, the method comprising the steps of:

(a) measuring a current value ΔM_(c) of the output parameter ΔM,

(b) estimating the relation between ΔM_(c) and ΔM_(o) by multiplyingΔM_(c) by a correction coefficient K which comprises an invariantcorrection coefficient component K_(o) inversely proportional to ΔM_(o),and

(c) determining a value F_(c) to which the input operation parameter Fshould be adjusted, as a function of KΔM_(c); characterized in that

(d) said correction coefficient K comprises a varying correctioncoefficient component whose current value K_(c) changes in accordancewith the variation of said operation condition B=B(t), the step (b)further comprising calculating the current value K_(c) and calculatingK=ƒ(K_(o), K_(c)).

Preferably, K=K_(o)−K_(c).

The operation input parameter F is preferably a feed rate of the turningtool and the operation output parameter ΔM is preferably a cuttingtorque developed by a drive rotating the workpiece. However, theoperation output parameter may also be a cutting force applied by thetool to the workpiece or a power consumed by the drive.

The predetermined value ΔM_(o) of the output parameter is preferably amaximal value ΔM_(max) which this parameter may have when the varyingoperation condition B differs to a maximal extent from its original ornominal value.

In accordance with preferred embodiments of the present invention, theinvariant correction coefficient component K_(o) is defined as${K_{o} = \frac{A}{\Delta \quad M_{\max}}},$

where ${A = \frac{F_{id}}{F_{o}}},$

with F_(id) being an idle feed and F_(o) being a pre-programmed basicfeed rate.

The varying operation condition B may be a real physical parameter suchas a depth of cut h_(c)=h_(c)(t), hardness of the workpiece material,etc., whereby current values of the varying coefficient component K_(c)may then be obtained based on sensing current values of this parameter.Alternatively, the varying operation condition B may be a mathematicalequivalent of one or more physical parameters of the cutting process.

In accordance with another aspect of the present invention, there isprovided an adaptive control system for adaptively controlling a turningoperation performed at a workpiece by a turning tool, by adjusting acontrolled input operation parameter F to maintain an output operationparameter ΔM substantially at a predetermined value ΔM_(o) and therebyto substantially compensate variation of said output operation parameterΔM caused by the variation of at least one operation condition B=B(t),the system comprising:

a sensor of the output operation parameter ΔM for providing a signalU_(c) proportional to a current value ΔM_(c);

an adaptive controller for determining a value F_(c) to which the inputoperation parameter F should be adjusted, as a function of kU_(c), wherek is a signal transmission coefficient which comprises an invariantsignal transmission coefficient component k_(o) inversely proportionalto ΔM_(o), said controller including an amplifier capable oftransforming the signal U_(c) into kU_(c); and

an input parameter override unit capable of being controlled by saidadaptive controller to adjust the controlled operation input parameterto F_(c);

characterized in that

said controller further comprises a correction processing means forcalculating k_(c)Uc_(c), where k_(c) is a varying signal transmissioncoefficient component whose current values depend on the variation ofsaid operation condition B=B(t), the controller being capable ofcalculating k=ƒ(k_(o),K_(c)).

Preferably, the adaptive controller is capable of calculatingk=k_(o)−k_(c), and calculating k_(o) as ${k_{o} = \frac{A}{U_{o}}},$

where U_(o) is a signal from the sensor of the operation outputparameter corresponding to the value ΔM_(o). Preferably, ΔM_(o)=ΔM_(max)and U_(o)=U_(max).

Preferably, the sensor of the output operation parameter ΔM is a sensorof a cutting torque developed by a drive rotating the workpiece and theinput parameter override unit is a feed rate override unit.

The correction processing means may comprise a sensor or a calculatorfor, respectively, sensing or calculating current values of theoperation condition B, to be subsequently used in the calculation ofk_(c).

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, preferred embodiments will now be described, by way ofnon-limiting examples only, with reference to the accompanying drawings,in which:

FIGS. 1A and 1B are block diagrams of adaptive control systems havingadaptive controllers in accordance with two different embodiments of thepresent invention;

FIGS. 2A and 2B illustrate physical models of the adaptive controllersshown, respectively, in FIGS. 1A and 1B;

FIG. 3 is a block diagram of a control system having a known adaptivecontroller;

FIG. 4 illustrates a physical model of the known adaptive controllershown in FIG. 3;

FIG. 5 illustrates the dependence of the cutting torque ΔM_(c) on thecutting depth h_(c) in systems having a known adaptive controller asshown in FIGS. 3 and 4 (curve I), and having an adaptive controlleraccording to the present invention (curve II).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A and 1B illustrate two different embodiments of an adaptivecontrol system according to the present invention, for use with aCNC-operated lathe for adaptively controlling a turning operationperformed on a workpiece by a cutting tool (not shown).

The control systems designated as 1 a and 1 b in respective FIGS. 1A and1B, each have a feeding means 2 connected to the cutting tool and aspindle drive 4 associated with the workpiece, that are instructed by aprogram of a CNC unit 6 to establish the relative movement between thecutting tool and the workpiece with pre-programmed values of respectivebasic feed rate F_(o) of the cutting tool and basic rotational speedS_(o) of the spindle.

Each control system, 1 a and 1 b further comprises a torque sensor 8 formeasuring a cutting torque ΔM_(c) developed by the spindle drive andvarying in time depending on a cutting depth h_(c)=h_(c)(t) andgenerating a signal U_(c) proportional to the cutting torque ΔM_(c). Italso has a feed rate override unit 9 for adjusting the feed rate F_(c)so as to maintain the cutting torque ΔM_(c) as close as possible to itsmaximal value ΔM_(max), required for the maximal metal-workingproductivity. The feed rate override unit 9 is controlled by an adaptivecontroller 10 operating on the signal U_(c) from the torque sensor 8 todetermine the extent F_(c)/F_(o) to which the override unit 9 shouldadjust the feed rate F_(c).

In accordance with the equation (1) presented in the Background of theInvention, the known adaptive controller of turning operations describedtherein determines F_(c)/F_(o) as:${\frac{F_{c}}{F_{o}} = {A - {k_{o}^{\prime}U_{c}}}},$

where k_(o)′ is a signal transmission coefficient of the known adaptivecontroller.

It will now be explained how in the adaptive controller 10 of thepresent invention, the signal transmission coefficient k, or itsphysical equivalent—the correction coefficient K− is calculated in amanner that takes into account the variation of the depth of cut h_(c).

As explained in the Background of the Invention, the cutting torqueΔM_(c) in turning operations may be expressed in accordance with theequation (4), in which, for the purpose of the present explanation, A isa coefficient characterizing the extent to which the feed rate F_(c) maybe increased relative to the pre-programmed value F_(o).

It follows from the equation (4) that, for ensuring the conditionΔM_(c)=ΔM_(max), the correction coefficient K should be: $\begin{matrix}{{K = {\frac{A}{\Delta \quad M_{\max}} - \frac{1}{c\quad F_{o}h_{c}}}},} & (7)\end{matrix}$

where in accordance with the present invention, A/ΔM_(max)=AK_(o)′constitutes a first correction coefficient component K_(o) which isinvariant in time and 1/cF_(o)h_(c) constitutes a second correctioncoefficient component K_(c) which varies in accordance with thevariation of the depth of cut h_(c).

Based on the equation (3)${\frac{1}{c\quad F_{o}h_{c}} = \frac{a_{c}}{\Delta \quad M_{c}}},$

wherefrom the correction coefficient K may also be expressed as:$\begin{matrix}{K = {\frac{A}{\Delta \quad M_{\max}} - {\frac{a_{c}}{\Delta \quad M_{c}}.}}} & (8)\end{matrix}$

It follows from the above that the second coefficient component K_(c)may be expressed either as 1/cF_(o)h_(c) or as a_(c)/ΔM_(c).

The determination of the correction coefficient K should be performedunder the logical conditions that K should not be less than a zero andshould not exceed 1/ΔM_(max).

FIGS. 2a and 2 b represent physical models of the determination of thecoefficient K, based on the above equations (7) and (8).

In the control systems 1 a and 1 b of the present invention, thephysical models presented in FIGS. 2A and 2B are implemented by theadaptive controller 10 constructed to determinekU_(c)=k_(o)U_(c)−k_(c)U_(c), where k_(o) is a predetermined invariantsign transmission coefficient component and k_(c) is a varying signaltransmission coefficient component dependent on the depth of cut h_(c).

The coefficient components K_(o) and K_(c) are determined in the samemanner as the correction coefficients K_(o) and K_(c). Namely, theinvariant coefficient component k_(o) is determined as${k_{o} = \frac{A}{U_{\max}}},$

where U_(max) is a signal from the torque sensor 8 corresponding to themaximal torque ΔM_(max). The varying coefficient component k_(c) isdetermined either as $\begin{matrix}{{k_{c} = \frac{1}{c\quad F_{o}h_{c}}},} & (9)\end{matrix}$

or, based on the equation (3), as $\begin{matrix}{k_{c} = {\frac{a_{c}}{U_{c}}.}} & (10)\end{matrix}$

To determine kU_(c), the adaptive controller 10 comprises an amplifier14 with the invariant signal transmission coefficient k_(o) and acorrection processing means 16 with the varying signal transmissioncoefficient k_(c). Depending on the manner in which the varying signaltransmission coefficient component k_(c) is determined (according toeither the equation 9 or the equation 10), the correction processingmeans 16 may have either a depth of cut sensor 20 a (FIG. 1a) or avariation calculator of cutting conditions 20 b (FIG. 1b), and acomputing element 22 for determining current values of k_(c)U_(c)respectively based on either equation (9) or equation (10) in accordancewith the respective physical models in FIGS. 2A and 2B.

By virtue of the adaptive control provided by the control system of thepresent invention, the feed rate of turning tools may be adjusted,taking into account the variation of the depth of cut h_(c), so as tomaintain the cutting torque ΔM_(c) as close as possible to its maximalvalue ΔM_(max), in a substantially wide range of the depth of cut,whereby the productivity of the metal-working is increased. This isillustrated in FIG. 5 as well as in the following table showingexperimental results obtained with a known adaptive control system andwith an adaptive control system according to the present invention:

h_(c)/h_(max): 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Known a_(I) 1.01.0 1.1 1.1 1.2 1.3 1.4 1.5 1.6 1.8 2.0 adaptive ΔM_(I) 1.0 0.90 0.800.72 0.70 0.62 0.50 0.43 0.30 0.20 0 Control ΔM_(max) system (I)Adaptive a_(II) 1.0 1.1 1.3 1.6 1.9 2.0 2.0 2.0 2.0 2.0 2.0 controlΔM_(II) 1.0 0.98 0.98 0.98 0.98 0.90 0.82 0.70 0.43 0.25 0 systemΔM_(max) of the present invention (II) Comparative (a_(II)/a_(I)-1)x 010 18 41 58 54 43 33 25 11 0 Productivity 100%

The above-described embodiments of the adaptive control system accordingto the present invention present non-limiting examples thereof, and itshould be clear to a skilled person that, within the scope of theclaims, this system may have features different from those described,and shown in the drawings.

What is claimed is:
 1. A method of adaptively controlling a turningoperation performed on a workpiece by a turning tool, by controlling anadjustable input operation parameter F of the movement of the turningtool relative to the workpiece, to maintain an output operationparameter ΔM substantially at a predetermined value ΔM_(o) and therebyto substantially compensate the variation of said output operationparameter ΔM caused by the variation of at least one operation conditionB=B(t) varying in time, the method comprising the steps of: (a)measuring a current value ΔM_(c) of the output parameter ΔM, (b)estimating the relation between ΔM_(c) and ΔM_(o) by multiplying ΔM_(c)by a correction coefficient K which comprises an invariant correctioncoefficient component K_(o) inversely proportional to ΔM_(o), and (c)determining a value F_(c) to which the input operation parameter Fshould be adjusted, as a function of KΔM_(c); characterized in that (d)said correction coefficient K comprises a varying correction coefficientcomponent whose current value K_(c) changes in accordance with thevariation of said operation condition B=B(t), the step (b) furthercomprising calculating the current value K_(c) and calculatingK=ƒ(K_(o),K_(c)).
 2. A method according to claim 1, whereinK=K_(o)−K_(c).
 3. A method according to claim 1, wherein the operationinput parameter F is a feed rate of the turning tool.
 4. A methodaccording to claim 1, wherein the operation output parameter ΔM is acutting torque developed by a drive rotating the workpiece.
 5. A methodaccording to claim 1, wherein the predetermined value ΔM_(o) of theoutput parameter is a maximal value ΔM_(max) which this parameter mayhave when the varying operation condition B differs to a maximal extentfrom its original or nominal value.
 6. A method according to claim 5,wherein the invariant correction coefficient component K_(o) is definedas ${K_{o} = \frac{A}{\Delta \quad M_{\max}}},$

where ${A = \frac{F_{id}}{F_{o}}},$

with F_(id) being an idle feed and F_(o) being a pre-programmed basicfeed rate.
 7. A method according to claim 1, wherein the varyingoperation condition B is a real physical parameter.
 8. A methodaccording to claim 7, wherein said parameter is the depth of cuth_(c)=h_(c)(t).
 9. A method according to claim 7, wherein current valuesof the varying coefficient component K_(c) are obtained based on sensingcurrent values of said parameter.
 10. A method according to claim 1,wherein the varying operation condition B is a mathematical equivalentof one or more physical parameters of the cutting process.
 11. Anadaptive control system for adaptively controlling a turning operationperformed at a workpiece by a turning tool, by adjusting a controlledinput operation parameter F to maintain an output operation parameter ΔMsubstantially at a predetermined value ΔM_(o) and thereby tosubstantially compensate variation of said output operation parameter ΔMcaused by the variation of at least one operation condition B=B(t), thesystem comprising: a sensor of the output operation parameter ΔM forproviding a signal U_(c) proportional to a current value ΔM_(c); anadaptive controller for determining a value F_(c) to which the inputoperation parameter F should be adjusted, as a function of kU_(c), wherek is a signal transmission coefficient which comprises an invariantsignal transmission coefficient component k_(o) inversely proportionalto ΔM_(o), said controller including an amplifier capable oftransforming the signal U_(c) into k_(o)U_(c); and an input parameteroverride unit capable of being controlled by said adaptive controller toadjust the controlled operation input parameter to F_(c); characterizedin that said controller further comprises a correction processing meansfor calculating k_(c)U_(c), where k_(c) is a varying signal transmissioncoefficient component whose current values depend on the variation ofsaid operation condition B=B(t), the controller being capable ofcalculating k=ƒ(k_(o),k_(c)).
 12. An adaptive controller according toclaim 11, further capable of calculating k=k_(o)−k_(c), and calculatingk_(o) as ${k_{o} = \frac{A}{U_{o}}},$

where U_(o) is a signal from the sensor of the operation outputparameter corresponding to the value ΔM_(o).
 13. An adaptive controlleraccording to claim 12, wherein ΔM_(o)=ΔM_(max) and U_(o)=U_(max).
 14. Anadaptive controller according to claim 11, wherein said sensor of theoutput operation parameter ΔM is a sensor of a cutting torque developedby a drive rotating the workpiece.
 15. An adaptive controller accordingto claim 11, wherein said input parameter override unit is a feed rateoverride unit.
 16. An adaptive controller according to claim 11, whereinsaid correction processing means comprises a sensor or for sensingcurrent values of the operation condition B, to be subsequently used inthe calculation of k_(c).
 17. An adaptive controller according to claim11, wherein said correction processing means comprises a calculator forcalculating current values of the operation condition B, to besubsequently used in the calculation of k_(c).